Conventional plasma etching of dielectric structures on workpieces has issues of reactive ion etch (RIE) lag (e.g., aspect ratio dependent etching), etch stop, or twisting of features etched. This is due in part to both competing polymer deposition mechanism using a fluorocarbon etch chemistry, and positive charging of the structure walls from predominantly positive ion fluxes and unequal electron fluxes down the depth of the structure. In addition, this charging leads to selectivity and damage issues, especially to the soft films encountered in dielectric etching. For example, some materials, such as insulating materials (e.g., dielectric materials) can charge during plasma etch processing. The charging of the material can result in damage, destruction, or over-etching of the material due to, for example, increased flux of a plasma species at a charged surface of the material.
Solutions to the surface charging problem include, for example, pulsing the RF source and bias frequencies to dissipate charge from the surface of the material. Typically, the pulsing of the RF source and bias signals is synchronized such that each RF signal is completely aligned, e.g., each signal is in phase and has the same duty cycle. However, the inventors have discovered that this type of synchronization does not effectively compensate for variations in the plasma due to the pulsing process which can result in a non-uniform plasma and a poorly controlled etch rate. The inventors have further discovered that this type of synchronization does not effectively compensate for a low ion density at initial plasma formation or the transition from an afterglow, e.g., where the RF source is in an “off” portion of the pulse period but some small plasma density remains, to an active glow, e.g., where the RF source is in an “on” portion of the pulse period. For example, at the leading edge of the “on” portion of the RF bias pulse period (i.e., synchronous with the leading edge of the “on” portion of the RF source pulse period), ions may attain high energies due to the application of the RF bias potential to an initially small plasma density at the leading edge of the “on” portion of the pulse period. Although these high energy ions arrive at the substrate for only about 2 to about 4 percent of the pulse period, the flux of the high energy ions is substantial enough to cause ion bombardment damage.
Accordingly, the inventors have provided improved methods for plasma processing.